Heretofore, a polyester resin such as a polyethylene terephthalate resin has been widely used as various packaging materials such as containers or films, or as fibers, etc., since it is excellent in mechanical strength, chemical stability, gas barrier property, hygienics, etc., and is relatively inexpensive and light in weight.
Such a polyester resin is produced mainly by using an antimony compound as a polycondensation catalyst. However, there has been concern about a problem such that the antimony compound or metal antimony remaining in the resin may elute, for example, in a step of being contacted with water for e.g. cooling after the polycondensation or in a step of being contacted with a solvent for e.g. dying after being processed into fibers, thus causing environmental pollution. Further, there has been concern about a possibility that in use as a packaging material for e.g. a container, it will elute from the container, for example, in a step of being contacted with hot water for e.g. heat sterilizing filling. Accordingly, various polyester resins have been proposed which are produced, for example, by using a titanium compound as a polycondensation catalyst instead of the antimony compound or using a titanium compound in combination therewith. However, there has been a problem such that the color tone of the polyester resin deteriorates, acetaldehyde, diethylene glycol, etc. will form, whereby the amount of such by-products in the polyester resin increases, or elution of antimony from the polyester resin cannot adequately be suppressed.
Meanwhile, when a polyester resin or the like is used for a hollow container for a beverage, it may be used, for example, for a non-carbonated beverage such as mineral water, tea or juice, or for a carbonated beverage. Further, irrespective of the non-carbonated or carbonated beverage, an unheated aseptic filling method and a heat sterilization filling method are available as methods for filling the beverage.
A conventional polyester resin obtainable by using an antimony compound as a catalyst has a high crystallization rate, whereby the transparency tends to be poor. Accordingly, especially when it is used for a hollow container for a non-carbonated beverage, it is common to suppress the crystallization rate to a proper level by copolymerizing e.g. diethylene glycol-isophthalic acid in a small amount and usually setting the molecular weight (usually represented by the intrinsic viscosity) of the resin to be relatively high, in order to form a container such as a bottle excellent in transparency. However, since a copolymer component is incorporated, there is a problem such that oriented crystallization will not adequately proceed at the time of molding, whereby it tends to be difficult to obtain a molded product having sufficient heat resistance and strength efficiently, and when formed into a molded product, the amount of by-products such as acetaldehyde contained in the molded product tends to increase. If the molecular weight is further increased, there has been a problem such that the productivity of the resin and the productivity in the molding tend to deteriorate, or the amount of by-products such as acetaldehyde tends to further increase.
Further, with a conventional polyester resin obtainable by using an antimony compound as a catalyst, the crystallization rate is high, and accordingly, it is common to carry out copolymerization of a corresponding amount of diethylene glycol as mentioned above, whereby the transparency when formed into a container may be improved, but in a case where it is used as a bottle particularly for a carbonated beverage, which is transported in a state where a stress is exerted by the inner pressure of the contained beverage, there has been a problem that cracks are likely to form by external factors such as the environmental temperature, chemical agents or solvents.
For the purpose of e.g. imparting environmental stress cracking resistance to a bottle for a carbonated beverage, a method of copolymerizing a polyfunctional compound component (e.g. JP-A-5-84808) or a method of applying anneal treatment to a bottle (e.g. JP-A-6-297550) has, for example, been proposed. However, such methods are not necessarily satisfactory from the viewpoint of the thermal stability during the production of a bottle, the transparency as a bottle or the productivity of the bottle.
Further, with a conventional polyester resin obtainable by using an antimony compound as a catalyst, the crystallization rate is so high that when a bottle obtained by molding it, is used particularly for heat sterilization filling, there has been a problem that deterioration of the transparency of a preform by heat treatment before blowing at the time of molding a bottle, tends to be remarkable. Accordingly, there have been many proposals from the viewpoint of the polycondensation catalyst, such as a method of using a titanium compound or a germanium compound, and further a magnesium compound and a phosphorus compound, in combination with the antimony compound, as a polycondensation catalyst. However, according to the study by the present inventors, it has been found that although in each proposal, the effect of lowering the crystallization rate is observed, there has been a problem that the above-mentioned heat treatment at the time of molding a bottle, takes time and there will be a difference between local crystallization rates, for example, between inside and outside of the mouth stopper portion, whereby the dimensional precision at the mouth stopper portion cannot be stabilized.
Further, with a conventional polyester resin obtainable by using an antimony compound as a catalyst, the crystallization rate is so high that there has been a problem that at the time of molding a bottle, in the injection molding of a preform, it is necessary to set the molding temperature at a high level for melting and plasticizing, followed by injection into a mold and by quenching in order to maintain the transparency, and the molding temperature is obliged to be high, consequently, by-products such as acetaldehyde, cyclic low molecular weight products, etc. will form in the resin after the molding, and such acetaldehyde may adversely affect the taste of the content when used as a bottle, or such cyclic low molecular weight products tend to contaminate the blow molding mold, whereby for the cleaning of the mold, the productivity will substantially be reduced.
Further, in order to solve the above-mentioned various problems, various proposals have been made for a process for producing a polyester resin wherein the amount of the antimony is reduced, and a titanium compound or a germanium compound, and further a magnesium compound and a phosphorus compound or the like are used in combination. However, by any one of conventional methods, it is difficult to sufficiently suppress elution of antimony, and there has been a problem that the above-mentioned other various problems cannot be adequately solved, or the polymerizability deteriorates, whereby the productivity of the polyester resin tends to be poor.
For example, JP-A-9-87374 discloses a process for producing a thermoplastic polyester, characterized in that in the production of a thermoplastic polyester resin comprising a dicarboxylic acid component and an alkylene glycol component, a mixture of an antimony compound and a titanium compound, and at least one compound selected from alkali metal compounds and alkaline earth metal compounds, are used as a polycondensation catalyst.
JP-A-2000-128964 discloses a polyester resin produced by using an antimony compound as a catalyst and containing ethylene terephthalate as the main repeating unit, which is characterized in that the haze of a molded product having a thickness of 4 mm molded from this resin at a temperature of 290° C., is not more than 3.0%, and the haze of a molded product having a thickness of 5 mm is not more than 15.0%.
Japanese Patent No. 03081104 discloses a polyester for forming a film comprising an aromatic dicarboxylic acid as the main acid component and an aliphatic glycol as the main glycol component, which is characterized in that the content of metal-containing precipitated particles by a catalyst used at the time of synthesizing the polyester, is not more than 0.01 wt % (based on the polyester), and the catalyst comprises a titanium compound or a titanium compound and an antimony compound, and the amounts of these metal elements satisfy the specific ranges.
JP-A-2000-219726, JP-A-2000-219730, JP-A-2000-226444, JP-A-2000-226445, JP-A-2000-226446 and JP-A-2000-226500 disclose polyester resins containing Sb and Ti or/and Ge as catalysts, and having densities and density-increasing rates within specific ranges.
However, according to the study by the present inventors, elution of antimony is not adequately suppressed, and the polymerizability and the productivity of the polyester resin are poor.
The present invention has been made in view of the above-described prior art, and it is an object of the present invention to provide a polyester resin poly-condensed in the presence of an antimony compound and having elution of antimony suppressed, and a process for producing a polyester resin, whereby such a polyester resin can be obtained with good polymerizability and productivity.